The present invention relates generally to golf clubs and, more particularly, to hybrid composite golf club shafts and methods of manufacturing the same.
With the advent of composite golf club shafts, it has become much easier to tailor the design of a golf club to the needs of a particular player or particular shot. For example, for longer shots and lower numbered irons, or woods, it is often desirable to use a more flexible shaft. Whereas, for shorter shots and higher numbered irons, it is often more desirable to use a stiffer shaft. Such design goals may be achieved, for example, through the use of additional layers of composite fiber in shorter shafted clubs and through the use of fewer layers of fiber in longer clubs. Such design goals may also be achieved by varying the orientation of the layers of composite fiber that make up a shaft. For example, to add stiffness to a club shaft it may be desirable to utilize several layers of composite fiber that run parallel to the longitudinal axis of the shaft, whereas to enhance the flexibility of a shaft it may be desirable to utilize several layers of composite fiber which are offset to a substantial degree, for example, +/-45.degree. or more, from the longitudinal axis.
With regard to the manufacture of composite golf club shafts, those skilled in the art will appreciate that each layer of composite fiber may be formed using a "pre-preg" composite sheet, and that pre-preg composite sheets may be manufactured by pulling strands of fiber, for example, carbon or glass fiber, through a resin solution and allowing the resin to partially cure. Exemplary resins or "binding matrices" may include, for example, thermoset epoxy resins and thermoplastic resins. Alternatively, pre-preg sheets may be manufactured by pulling a fabric or weave of composite fabric through a resin solution and allowing the resin to partially cure. In either case, once the resin is partially cured or "staged," the resin holds the fibers together such that the fibers form a malleable sheet.
It also will be appreciated that the performance characteristics of a golf club shaft may be significantly altered through the choice of resin or binding matrix used within the shaft. For example, it will be appreciated that composite materials bound within a thermoset binding matrix may be used to produce stiffer shafts, whereas similar composite materials bound within a thermoplastic resin may produce a softer feeling shaft. Further, it will be noted that shafts manufactured from thermoset materials are generally harder and more brittle than shafts manufactured from thermoplastic materials, and that thermoplastic materials provide a greater amount of vibration damping than do thermoset materials when used within a golf club shaft. Those skilled in the art also will appreciate that better impact tolerance characteristics may be achieved through the use of thermoplastic materials as opposed to thermoset materials.
Prior to this time, however, those skilled in the art have encountered substantial difficulty in developing a hybrid golf club shaft that provides the benefits associated with both thermoplastic and thermoset resin pre-pregs. One reason for this is that, prior to the conception of the subject invention, difficulty was encountered in providing a durable bond between the layers of thermoplastic and thermoset resin within a golf club shaft. Moreover, when attempts were made to produce a hybrid thermoplastic/thermoset golf club shaft, substantial issues of durability were encountered as a result of sheering between the respective layers. Stated somewhat differently, when attempts were made to produce hybrid thermoplastic/thermoset golf club shafts using conventional methodologies, the resulting shafts were not able to pass standard durability requirements. For example, when fitted with club heads and swung by a mechanical ball-hitting machine, the shafts were unable to withstand 550 high heel hits without incurring substantial delamination of the resin layers comprising the shafts.
In view of the foregoing, it is submitted that those skilled in the art would find an improved hybrid thermoplastic/thermoset composite golf club shaft capable of passing rigorous durability testing, and methods of manufacturing such shafts, to be quite useful.